The present invention relates to a novel photochemical gas-liquid process for halogenating alkylbenzenes, particularly a process for chlorination of toluene.
Photochemical chlorination, or in broader terms halogenation, of hydrocarbon derivatives is currently employed on a wide scale. Nevertheless, in reactions involving substituting hydrogen atoms by chlorine atoms, numerous problems of selectivity occur. One representative example of photochemical chlorination is the chlorination of alkylbenzene, particularly toluene.
Photochemical chlorination of toluene principally leads to the formation of benzyl chloride, benzylidene chloride and phenylchloroform.
It is also known that apart from this principal chlorination of the methyl group of the toluene molecule, unwanted small amounts of chlorinated derivatives on the aromatic ring always form, particularly chlorotoluenes and homologous compounds thereof: benzyl chloro-chlorides, benzylidene chloro-chlorides and chlorophenylchloroforms.
It is moreover known that the proportion of chlorine derivatives on the aromatic ring increases substantially as the degree of chlorination of the mixture increases. Starting from 0.1 to 0.5%, when limited to benzyl chloride, the proportion of chlorine derivatives on the aromatic ring can reach or even exceed 5% when it is desired to achieve complete chlorination of the methyl group, for the purposes of producing essentially phenylchloroform.
For this reason, it is difficult to obtain high-purity phenylchloroform by direct chlorination without having recourse to distillation of the reaction mixture. Certain patents hence consider carrying out incomplete chlorination in order to limit the formation of derivatives, and to then subsequently distil the mixtures obtained with a subsequent increase in cost price.
Numerous photochemical reactors have been developed, some of them for a specific use. Thus JP-A-85 25153 discloses a benzene chlorination reactor, but this chlorination reaction does not involve in any critical way the problems of selectivity listed above for toluene. This reactor is a special reactor, having fluid convection. The vertical cylindrical reaction vessel has an internal cylindrical wall with a mirror surface. An internal fluid circulation is achieved by means of a rising flow inside the cylinder, and a descending flow between the outer casing of said cylindrical wall and the said internal wall. The reaction vessel is fed with gas from the bottom, and the light source is disposed inside the reactor.
The documents xe2x80x9cReaktionsapparate fxc3xcr Gas-Flxc3xcssig-Reaktionenxe2x80x9d, p. 372, and xe2x80x9cChemischtechnisches Lexikonxe2x80x9d describe reactors of the type convection reactor using the air-lift principle and/or which include an entraining flow which is injected at the base of the apparatus. The example of implementation concerns oxidation of n-butane.
None of these documents discloses the process according to the present invention.
The present invention provides a photochemical gas-liquid process for halogenating alkylbenzenes enabling high yield and high selectivity to be achieved together with other advantages.